MODELING THE EFFECT OF DEFORMATION-INDUCED VACANCIES ON SEGREGATION AND PRECIPITATION

The influence of deformation-induced excess vacancies on segregation and on precipitation is discussed within a phenomenological framework. During high temperature deformation, vacancies are mainly generated by the non-conservative movement of thermal jogs. Sinks for these vacancies are dislocations and grain boundaries. The excess vacancy concentration increases with strain rate and inverse temperature. Stronger effects are expected in fc.c. materials because of their comparatively low diffusivities; in austenitic steels, the concentrations of deformation-induced vacancies are predicted to be significant, even at temperatures as high as 0.75 of the melting temperature. At the high strain rates ( > 10 s-1) used in industrial rolling, the vacancy concentration may attain levels 2-3 orders-of-magnitude higher than the equilibrium one. One consequence of the presence of deformation-induced vacancies is the non-equilibrium segregation of boron observed e.g. in austenitic Nb-B steels, where it stimulates the nucleation of Nb borocarbonitrides.